FR2757319A1 - EMISSION FREQUENCY MICROLASER MODULATED WITH A PIEZOELECTRIC OR ELECTRO-OPTIC ELEMENT - Google Patents

Abstract

The invention concerns a micro-laser resonator comprising: a solid active medium (2) with piezoelectric or electro-optic properties; an input mirror (4); an output mirror (6); means (8, 10) for applying to the solid active medium a longitudinal electric field. In another embodiment, the micro-laser resonator comprises a solid active medium and an element made of a piezoelectric or electro-optic material, means being provided for applying to this element a longitudinal electric field.

Description

TRANSMIT FREQUENCY MICROLASER MODULATED USING A
PIEZOELECTRIC OR ELECTRO-OPTICAL ELEMENT
DESCRIPTION
Technical area
The invention relates to the field of solid microlasers.

 A microlaser has a structure which consists of a stack of multilayers. The active laser medium consists of a material of small thickness (for example between 150 and 1000 μm) and of small dimension (twisting section of a few mm 2) r on which dielectric cavity mirrors are directly deposited. This active medium can be pumped by a III-V laser diode which is either directly hybridized on the microlaser, or coupled to the latter by optical fiber.

 Microlasers have many applications, in fields as varied as the automotive industry, the environment, scientific instrumentation or telemetry.

For a sufficiently short length of the Fabry-Perot cavity of the microlaser, it is possible to obtain a single laser emission line, while the transverse structure of the beam is close to the theoretical TEMoo mode (the beam is circular and Gaussian)
Modulated emission frequency microlasers find applications in the field of chemical analysis of gaseous constituents, analysis of velocity fields, non-contact metrology, laser imaging, or medical instrumentation. .

 Various techniques are known for tuning, or modulating, the laser emission wavelength.

According to a first technique, described for example in the article JJ Zayhowski entitled "Microchip Lasers", published in The Lincoln Laboratory
Journal, vol. 3, n "3, page 427 et seq., 1990, a piezoelectric element makes it possible to apply a transverse force on the structure of the microlaser, which generates a modulation of the optical frequency of a few MHz, for a modulation recurrence ranging from continuous at 40 ns. The voltage applied to the terminals of the piezoelectric element is between -1000 V and + 1000 V, which poses problems of electromagnetic compatibility. This also requires finding sophisticated electronics incompatible with simplicity and low cost. microlasers production.

Another technique consists in introducing an electro-optical element into the microlaser cavity: such a technique is described for example in the article
JJ Zayhowski et al. Title: "Diode pumped composite cavity electro-optically tuned microchip
Laser ", published in IEEE Photonics Technology Letters, vol. 5, ne10, pp. 1153, October 1993. The structure described in this document requires the bonding of an electro-optical material on the active laser medium. Electrodes are arranged in vis-à-vis in the direction of the crystallographic axis C of the electro-optical medium An excursion of the optical frequency of 30 GHz was observed by applying a voltage varying from -1000 V to +1000 V on the electrooptic crystal. This type of device presents difficulties of implementation. It is difficult to make all the elements cohabit, for problems of electromagnetic compatibility. In addition, the large value of voltage variation necessary for the operation of the device requires sophisticated electronics incompatible with the simplicity and low cost of production of the microlaser.

Other techniques have been proposed for obtaining frequency modulation. For example, modulation of the optical frequency can be obtained by modulating the power of the pumping beam: this technique is described in the article by JJ Zayhowski et al. entitled "Frequency tuning of microchip lasers using pump-power modulation" published in IEEE J. Of
Quantum Electronics, vol. 28, n "4, p. 1118, April 1992. The main drawback of this modulation process comes from the fact that amplitude modulation accompanies frequency modulation.

This amplitude modulation is undesirable in most applications. In addition, the amplitude of the frequency sweep is low, because it is limited by the temperature variation of the amplifier crystal created by the variations in the pump power.

Statement of the invention
The subject of the invention is a microlaser structure comprising frequency modulation means, and having a high modulation efficiency.

The subject of the invention is a microlaser cavity comprising - a solid active medium having properties
piezoelectric or electro-optical, - an input mirror, - an output mirror, - means for applying to the solid active medium a
longitudinal electric field.

 In such a structure, which is both monolithic and of small dimensions, the optical path in the laser cavity is modified by application, to the solid active medium, of the longitudinal electric field. The latter makes it possible to modify the piezoelectric or electro-optical properties of this active medium.

 The term “longitudinal field” is understood to mean a field directed along the pumping and emission axis of the microlaser cavity.

 Such a structure makes it possible to obtain a greater modulation efficiency than in the devices known according to the prior art. In particular, for piezoelectric type structures, operating at resonance, mechanical resonance can be obtained with low control voltages (from 1 to 100 volts). At resonance, longitudinal deformations of the cavity of several hundred nanometers are possible.

This causes a frequency variation of several tens of Gigahertz (typically from 1 to 100 GHz).

In addition, the application of a longitudinal electric field does not affect the transverse structure of the laser beam. On the contrary, most devices using the electro-optical effect implement the transverse electro-optical effect; this is the case, for example of the device described in the article by
Zayhowski et al. entitled "Diode pumped composite cavity electro-optically tuned microchip laser", IEEE
Photonics Technology Letters, vol. 5, n "10, p. 1153,
Oct. 1993. In this type of structure certain problems have been observed: in particular there is a residual piezoelectric effect, difficult to eliminate, which alters the purity of the TEMoo laser mode. There is an appearance of parasitic transverse modes. This effect, which results in laser power instability and also reduces the coherence length of the laser, does not appear in a device according to the invention.

 In the microlaser according to the invention, it is possible to work with low control voltages to modulate the emission frequency of the laser: this device therefore compares favorably from the point of view of electromagnetic compatibility, with the devices of the prior art . For example, in the piezoelectric case, at resonance, the control voltages range from 1 to 100 volts.

Finally, in such a device, there is no change in polarization of the laser beam for dynamic operation. Conventionally, when the field is applied according to the area C of the crystal, assumed to be uniaxial, the wave propagates in a medium whose birefringence is written, for a crystal of symmetry 3m:

 This is the case for LiTaO3 and LiNbO3. The modulation of the birefringence can induce a polarization shift if the E field is too large.

In the invention, on the contrary, this is theoretically not possible since the application of the field E does not modify the state of birefringence of the crystal, provided that the axis C of the crystal is placed according to the direction of propagation of the light, the field E being itself applied along the axis C
The means for applying a longitudinal electric field to the solid active medium may comprise first and second electrodes, arranged at the inlet and at the outlet of the microlaser cavity.

 In particular, these electrodes may each comprise a layer of conductive material, formed on one of the faces of the microlaser cavity.

 The material constituting such a layer preferably has micro-optical properties of transparency and reflectivity adapted to the face of the cavity of the microlaser on which it is deposited.

 Such a material can be ITO, or indium oxide, or tin oxide doped with antimony or fluorine, or zinc oxide doped with aluminum, gallium or indium.

 At least one of the two layers of conductive material can be deposited between one of the two input and output mirrors of the microlaser cavity, and the active laser medium.

 Metal contacts can be formed on each electrode: this makes it possible to promote a homogeneous distribution of the potential and makes it possible to obtain a symmetrical deformation of the structure.

The active laser medium can be
(LalxNdx) 3Gassiola and, for example, La3GasSiOl4, which have piezoelectric properties.

 It can also be Nd: LiTaO3: MgO, which has electro-optical properties.

According to another embodiment, a microlaser cavity according to the invention comprises - a solid active medium, - an input mirror, - an output mirror, - an element made of a piezoelectric or electro material
optical, - means for applying to the piezoelectric element
or electro-optic a longitudinal electric field.

 This second embodiment has the same advantages as the first: compared with known devices, the modulation efficiency is greater and the control voltages to be applied are lower. In addition, this structure does not affect the transverse structure of the microlaser and there is no change in polarization of the beam in dynamic operation.

 Here again, the term “longitudinal field” is understood to mean a field directed along the pumping and / or emission axis of the microlaser cavity.

 In this second embodiment, the means for applying a longitudinal electric field to the piezoelectric or electrooptical element may comprise first and second electrodes arranged on either side of said element.

 Thus, the first electrode can be located between the active laser medium and the electrooptic or piezoelectric element, and comprises a layer of conductive material transparent to the wavelength of the laser beam. This electrode can be for example ITO.

Advantageously, it is possible to have between the active laser medium and the piezoelectric element, or electro-optic, a film of adhesive of index substantially equal to 2 and of thickness substantially equal to a multiple of S / 4, where k denotes the emission wavelength of the microlaser: these conditions make it possible to obtain negligible parasitic reflectivities at the interface between the solid active medium and the piezoelectric or electro-optical element in the case of the
YAG: Nd / LiTaO3.

 A second electrode can be arranged on an external face of the microlaser cavity, and be provided with a hole to let the laser beam pass.

Brief description of the figures
In any case, the characteristics and advantages of the invention will appear better in the light of the description which follows. This description relates to the exemplary embodiments, given by way of explanation and without limitation, with reference to the appended drawings in which
FIG. 1 represents a first embodiment of the invention,
FIG. 2 represents a variant of the first embodiment of the invention,
FIGS. 3A to 3D illustrate steps of a method for producing a device according to the invention,
- Figures 4 and 5 show another embodiment of the invention.

Detailed description of embodiments of the invention
A first embodiment of the invention will be described in connection with FIG. 1. In this figure, the reference 2 designates an active laser medium which forms, with input and output mirrors 4, 6 a microlaser cavity.

This medium has piezoelectric or electro-optical properties. Various laser arrays can exhibit such effects and are available on the market. For example, La3GasSiO14 has a piezoelectric effect, while the
Nd: MgO: LiNbO3 can be used to exploit the electro-optical effect. Furthermore, all piezoelectric or electro-optical matrices can receive an active laser ion and be used as an active laser medium.

Electrodes 8, 10 are arranged on either side of the microlaser cavity. In fact, in the example given, these electrodes are produced in the form of thin layers on the input and output mirrors 4, 6 of the microlaser cavity. These electrodes allow a longitudinal electric field to be applied
E inside the microlaser cavity. This electric field is directed substantially along the axis of the cavity, that is to say along the axis of propagation of a pumping beam 14 of the active laser medium and the laser beam 16 emitted by the microlaser. The electrodes 8, 10 are connected, via contacts 18, 20 to means for supplying an appropriate voltage. These means are not shown in the figures. The application of the electric field E makes it possible to use the electro-optical or piezoelectric properties of the active laser medium 2.

 In FIG. 1, the reference 12 designates a diode for pumping the active laser medium.

 Preferably, the thin layers 8, 10 have optical properties adapted to the faces of the microlaser on which they are deposited.

Thus, if the layer 8 is deposited on the side of the microlaser through which the pumping beam 14 enters, this layer will be transparent, that is to say will present a certain degree of transparency to the pumping beam.

In addition, it can be transparent at the wavelength of the laser. For the layer 10, deposited on the other side, this is at least transparent, that is to say has a certain degree of transparency, at the emission wavelength of the laser beam 16.

 The materials capable of fulfilling these conditions are for the majority of oxides, the most common of which are indium oxide, indium oxide doped with tin (ITO), tin oxide doped with l antimony or fluorine, zinc oxide doped with aluminum, or gallium, or indium.

The metal contacts 18, 20 are preferably deposited on each electrode 8, 10 to promote a homogeneous distribution of the potential in the electrode. This makes it possible to obtain a symmetrical deformation of the structure
FIG. 2 represents a variant of this first embodiment: numerical references identical to those of FIG. 1 designate identical or corresponding elements therein. In FIG. 2, the electrodes for controlling the electric field inside the active laser medium or the microlaser cavity are deposited directly in contact with the active laser medium, and the mirrors 4, 6 are deposited on these layers of electrodes 8, 10. The transparency and / or reflectivity properties of the layers 8, 10 are the same as those set out above, depending on the side (inlet or outlet of the microlaser cavity) on which they are located.

 The embodiment explained above, in these two variants, has the advantage of a compact structure compared to the device using on the one hand an active laser medium and on the other hand an electro-optical or piezoelectric element . In addition, such a structure is compatible with a collective production of a microlaser, which is not the case of the piezoelectric structures described for example in the article by J.J Zayhowski entitled "Laser microchip" already cited above. Indeed, in this structure of the prior art, the assembly of the two components of the microlaser results in a high manufacturing cost and, moreover, the reproducibility of the performances is not ensured from one microlaser to another.

 On the contrary, the device according to the invention at a low manufacturing cost and the implementation of a collective manufacturing process ensures the reproducibility of the performances.

 A method of producing a microlaser according to the invention will now be described, in conjunction with FIGS. 3A to 3D.

 A first step (FIG. 3A) consists in choosing the active laser material, in orienting it and in cutting it into strips 30, of thickness ranging for example from a few hundred micrometers to a few millimeters. These blades are then lapped and polished.

 If we are dealing with a piezoelectric material, it is preferably oriented so that longitudinal waves parallel to the axis C (axis of the laser cavity, are excited. If we are dealing with an electro-optical material (variation of the refractive index), the crystallographic axis C will also preferably be oriented along the axis of the laser cavity, this in particular to avoid polarization tilting, but this is not an imperative and others configurations can be imagined depending on the exact nature of the material used.

 The layers 32, 34 of input and output mirrors are then deposited. These are, for example, dichroic mirrors, obtained by depositing dielectric multilayers.

 Then (Figure 3B) is deposited on the layers 32 and 34, layers of a conductive material. Then, by mechanical masking and etching by "lift-off", these layers are engraved so as to determine zones 36, 38, 40, 42 adapted to individual microlasers.

 For the production of a structure of the type of that of FIG. 2, the order of deposition of the mirror layers and of the electrode layers is reversed.

 In a third step (FIG. 3C), contact points 44, 46, 48, 50, 52, 54, 56, 58 are made for each conductive layer 36, 38, 40, 42.

 The last step (Figure 3D) is a step of cutting the individual microlaser cavities. Each cavity then comprises an active laser medium, its input and output mirrors, its conductive layers and its contact pads.

 The assembly can then be coupled to a pumping diode directly hybridized to the microlaser cavity or connected to it by an optical fiber.

 Another embodiment of the invention will be described in connection with FIGS. 4 and 5.

 In these figures, the reference 62 designates the active medium of the microlaser. The microlaser cavity is limited on the one hand by an input mirror 64 and on the other hand by an input mirror 66. It further comprises an element 68 having piezoelectric or electro-optical properties. The properties of this material are controlled with electrodes 70, 72 which make it possible to establish a longitudinal electric field there: the longitudinal field lines are represented, inside the medium 68, in FIG. 5. The field is therefore oriented along an axis parallel to the direction of the laser beam 86 emitted by the microlaser and / or along the direction of the pumping beam of the microlaser. By contacts 71, 73 the electrodes 70, 72 are connected to voltage control means. The entire structure is pumped using a laser diode 80 which emits a pumping beam 82. This beam is collimated and focused using means 84 for collimation and focusing in the amplifying medium 62.

 According to the embodiment shown, the electrode 70 is located between the active laser medium 62 and the medium 68. It essentially consists of a layer of conductive material which is transparent at the wavelength of the laser beam.

It is not necessarily transparent to the pumping wavelength of the active medium: it can absorb or reflect the pumping power. The reflection is however favorable. In a particular embodiment, ITO can be used.

Such a material has the advantage of having a low resistivity, of the order of 2.10-3 to 5 5.10 3 Qm more, another advantage of this material, in the case where the active medium 62 is YAG: Nd, is to have a refractive index close to that of the amplifying material. Indeed, typically, we have - niTc = 1.80, - nyAG = 1.82.

 Thus, with such indices, the difference nITO-nyAG ~ 0.02 is small, which makes it possible to achieve a good impedance adaptation between the two media. In addition, still in the case of ITO, the specific absorption of electrode 70 is low, since the imaginary part K of the refractive index is of the order of 10-2 to 1.064 pm, where K is such that: n = nIT-iK.

 This keeps a reasonable operating threshold for the microlaser.

 As for the electrode 72, it can for example be made of a conductive material such as copper, and be provided with a hole 73 in its center, which allows the laser beam 86 emitted by the microlaser cavity to pass through.

 One can for example impose on the electrode 70 a potential V (t), while the electrode 72 is maintained at a zero potential.

 In Figure 5, there are shown, inside the middle 68, a few lines of the applied longitudinal field. The combination of electrodes 70, 72 forms a sort of capacitor in which is housed the piezoelectric or electro-optical material 68 (for example made of LiNbO3 or LiTaO3).

The material constituting the active laser medium 62 can be doped with neodymium (Nd) for a laser emission around 1.06 μm. This material can be chosen, for example, from one of the following materials: YAG
(Y3Al5O12), LMA (LaMgAl11O19), YV04, YSO (Y2SiOs), YLF
(YLiF4) or GdVO4, etc.

For emissions at other wavelengths, different materials and dopants will be chosen. In general, the active ions are chosen from - Nd for an emission around 1.06 pm, - Er or an erbium-ytterbium codoping Er + Yb for a
emission around 1.5 µm, - Tm or Ho or a coding of thulium and holmium for
an emission around 2 µm.

Furthermore, on the thickness e of the active medium depend certain characteristics of the microlaser - on the one hand, the absorption of the pump beam is
the greater the greater the thickness e; in
effect, if 1o is the intensity of the incident pump
on the input face, and "a" the coefficient
absorption, the intensity absorbed over the thickness e
will be given by
I absorbed = Io (le-ae), - on the other hand, the number of longitudinal modes of a
Fabry-Pérot cavity increases with thickness and, if
we want to make a longitudinal single-mode laser,
this thickness should preferably be small. In
effect, in a Fabry-Pérot cavity plan-plan
of thickness e, the free spectral interval dv between
two Fabry-Pérot modes is given by
dv = c / 2e
with c: speed of light.

If dg is the width of the gain band
(laser emission) of the material, the number of modes N will be given by
N = dg / dv
We see that, for a given material (a and dg given), when e increases, the absorption of the pump increases but N also increases. For a single-frequency laser, the minimum thickness is generally chosen for N = l. Typical thicknesses to obtain a single mode are - YAG L = 750 µm, - YVO4 L = 500 m, - LMA L = 150 µm.

 The input and output mirrors 64, 66 of the microlaser cavity are of the multilayer type. The input mirror 64 has for example a maximum reflectivity (r> 0.99) for k = 1.064 μm in the case of an active medium YAG: Nd. In the case of another active medium, the mirror 64 has a reflectivity r> 0.99 to?, Where XL is the fluorescence transition of the active laser medium, and a maximum transmission (t> 0.20) at length. pumping wave Bp. In the case of YAG: Nd, we preferably have # p # 0.810 μm.

 The output mirror 66 typically has a reflectivity r> 0.99 at the pumping wavelength Xp, and a transmission approximately equal to 0.05 at XL (for YAG Nd, # o # 0.810 m and # L = 1.064, um).

The device according to the invention may also comprise a layer 74 which is a film of adhesive whose refractive index is chosen so as to minimize the reflectivity of the interface between the active laser medium 62 and the element 68. By example, in a crystal
Nd: YAG by introducing an adhesive film of index equal to, or substantially equal to 2 and of thickness equal to, or substantially equal to an integer multiple of # L / 4, parasitic reflectivities rp <1.5.10 are obtained -4, which is negligible. It is also possible to use an adhesive film having a refractive index other than 2 (typically, close to 1.5). In this case, it is preferable to add to the interface 62-68 a complementary layer carrying out the adaptation of optical impedance, in order to minimize the reflectivity at this interface.

The emission frequency of the microlaser whose structure has just been described is given by the formula
vs
VL = q 2n e (1) where q is an integer, c is the speed of light in a vacuum and nl is the equivalent optical path in the laser cavity.

For a small variation of the equivalent optical path in the cavity we have
AVL q. q- 2 = = q2 2 (tAn + nt () (2)
2n2 f2 2n212
If, for example, the longitudinal piezoelectric effect of lithium niobate (LiNbO3) is used, the longitudinal electro-optical effect is negligible compared to the piezoelectric effect when the structure is excited in a close mode.
in a resonant fashion. We can then neglect, in
equation (2), the term fAn with respect to n # l
tVL = VL Af = Af (3)
f. hL f
According to a numerical example, for t = 10-3 m,
c = 3.108 m / s and R = 1.064.10-6 m (case of YAG: Nd), we have a
sensitivity 6 = Av - c C - 28.2 MHz / nm. This example
Af
explains the advantage of having a type modulation
piezoelectric: for a variation of the length of
cavity of about 10 nm we have a frequency variation of about 2.82 GHz.

A method of producing a microlaser as illustrated in FIGS. 4 and 5 may include the following steps - selection, cutting and polishing of a blade of material
laser, - deposition of an input mirror layer (for example
dichroic mirror, obtained by depositing multilayers
dielectric), - deposition of a layer of material to form
the intermediate electrode 70, - bonding of a plate of electro-optical material pu
piezoelectric, - deposition of an output mirror layer (mirror
dichroic, for example), - cutting and mounting an output electrode for
each individual microlaser, - coupling with a pumping diode.

Claims (14)

 active solid a longitudinal electric field.  piezoelectric or electro-optical, - an input mirror (6), - means (8, 10, 18, 20) for applying to the medium  l Microlaser cavity comprising - a solid active medium (2) having properties  2. Microlaser cavity according to claim 1, the means for applying to the solid active medium a longitudinal electric field comprising a first  (8) and a second (10) electrodes arranged at the inlet and at the outlet of the microlaser cavity.  3. Microlaser cavity according to claim 2, the first and second electrodes each comprising a layer of conductive material, formed on one of the faces of the microlaser cavity, this layer having optical properties of transparency and reflectivity adapted to the face of the cavity on which it is deposited.  4. Microlaser cavity according to claim 3, the conductive material being ITO or indium oxide or tin oxide doped with antimony or fluorine, or doped zinc oxide aluminum gallium or indium.  5. Microlaser cavity according to one of claims 3 or 4, at least one of the two layers of conductive material being deposited between one of the two input and output mirrors of the microlaser cavity and the active laser medium.  6. Microlaser cavity according to one of claims 2 to 5, metal contacts (18, 20) being formed on each electrode.  7. Microlaser cavity according to one of the preceding claims, the laser active medium being of general formula (Lal-xNdx) 3GassiOlqt for example La3Ga3SiOla or Nd: LiTaO3: MgO.  8. Microlaser cavity, comprising - a solid active medium (62), - an input mirror (64), - an output mirror (66), - an element made of a piezoelectric or electro material  optics (68), - means (70, 72) for applying to the element  piezoelectric or electro-optic a field  longitudinal electric.  9. Microlaser cavity according to claim 8, the means for applying to the piezoelectric or electro-optical element a longitudinal electric field comprising first and second electrodes (70, 72) disposed on either side of said element.  10. Microlaser cavity according to claim 9, the first electrode being located between the active laser medium (62) and the electro-optical or piezoelectric element (68), and comprising a layer of transparent conductive material at a wavelength of laser beam.  11. Microlaser cavity according to claim 10, the electrode (70) located between the active laser medium (62) and the piezoelectric element (68) being made of ITO.  12. Microlaser cavity according to claim 11, the active laser medium (62) being YAG doped with neodymium ions Nd3 +.  13. Microlaser cavity according to one of claims 10 to 12, the second electrode (72) being arranged on an outer face of the microlaser cavity and being provided with a hole (73) for letting the laser beam pass.  14. Microlaser cavity according to one of claims 8 to 13, the solid active medium (62) being doped with neodymium ions, or erbium, or with an erbium-ytterbium coding, or with thulium or holmium ions or with a coding thulium-holmium.